Datasheet

6
LTC1569-6
BLOCK DIAGRA
W
10TH ORDER
LINEAR PHASE
FILTER NETWORK
POWER
CONTROL
DIVIDER/
BUFFER
R
EXT
PRECISION
OSCILLATOR
5
6
7
8
4
3
2
1 OUT
V
+
R
X
DIV/CLK
IN
+
IN
–
GND
V
–
1569-6 BD
APPLICATIONS INFORMATION
WUU
U
Self-Clocking Operation
The LTC1569-6 features a unique internal oscillator which
sets the filter cutoff frequency using a single external
resistor . The design is optimized for V
S
= 3V, f
CUTOFF
=
64kHz, where the filter cutoff frequency error is typically
<1% when a 0.1% external 10k resistor is used. With
different resistor values and internal divider settings, the
cutoff frequency can be accurately varied from 1kHz to
64kHz. As shown in Figure 1, the divider is controlled by
the DIV/CLK (Pin 5). Table 1 summarizes the cutoff
frequency vs external resistor values for the divide-by-1
mode.
LTC1569-6
18
27
36
45
DIVIDE-BY-4
DIVIDE-BY-1
DIVIDE-BY-16
V
+
V
–
R
EXT
100pF
f
CUTOFF
=
64kHz (10k/R
EXT
)
1, 4 OR 16
1569-6 F01
IN
+
IN
–
GND
V
–
OUT
V
+
R
X
DIV/CLK
Figure 1
Table1. f
CUTOFF
vs R
EXT
, V
S
= 3V, T
A
= 25°C, Divide-by-1 Mode
R
EXT
Typical f
CUTOFF
Typical Variation of f
CUTOFF
3844Ω* N/A ±3.0%
5010Ω* N/A ±2.5%
10k 64kHz ±1%
20.18k 32kHz ±2.0%
40.2k 16kHz ±3.5%
*R
EXT
values less than 10k can be used only in the divide-by-16 mode.
In the divide-by-4 and divide-by-16 modes, the cutoff
frequencies in Table 1 will be lowered by 4 and 16
respectively. When the LTC1569-6 is in the divide-by-4
and divide-by-16 modes the power is automatically re-
duced. This results in up to a 40% power savings.
The power reduction in the divide-by-4 and divide-by-16
modes, however, effects the fundamental oscillator fre-
quency. Hence, the effective divide ratio will be slightly
different from 4:1 or 16:1 depending on V
S
, T
A
and R
EXT
.
Typically this error is less than 1% (Figures 4 and 6).
The cutoff frequency is easily estimated from the equation
in Figure 1. Examples 1 and 2 illustrate how to use the
graphs in Figures 2 through 7 to get a more precise
estimate of the cutoff frequency.
Example 1: LTC1569-6, R
EXT
= 20k, V
S
= 3V, divide-by-16
mode, DIV/CLK (Pin␣ 5) connected to V
+
(Pin 7), T
A
= 25°C.